1. Field of the Invention
The present invention relates to methods and apparatus for mitigating rain fading over satellite communications (SATCOM) links using information throughput adaptation and, more particularly, to techniques for maximizing information throughput while maintaining acceptable SATCOM link fade margins under varying rain fade conditions over a plurality of communications links.
2. Description of the Related Art
As terrestrial communications systems and satellite communications systems evolve, certain trends present an opportunity to combine terrestrial and satellite infrastructures into broadband communications systems that take advantage of these evolutions to provide enhanced capabilities and performance.
With regard to terrestrial communications, terrestrial infrastructures are evolving from a circuit-switched architecture, such as that used in conventional voice telephone circuits, to a packet-switched, shared-media architecture. Packetized communications networks typically format data into packets for transmission from one site to another. In particular, the data is partitioned into separate packets at a transmission site, wherein the packets usually include headers containing information relating to packet data and routing. The packets are transmitted to a destination site in accordance with any of several data transmission protocols, such as Asynchronous Transfer Mode (ATM), Frame Relay, High Level Data Link Control (HDLC), X.25, IP, etc.), by which the transmitted data is restored from the packets received at the destination site.
Packetized data communications are appealing for common carrier or time-shared switching systems, since a packet transmission path or circuit is unavailable only during the time when a packet utilizes the circuit for transmission to the destination site, thereby permitting other users to utilize that same circuit when the circuit becomes available. Each individual transmission circuit and access channel connecting an end user to the packet switching network has a maximum data carrying capacity or bandwidth that is shared among the various users of the network. The access channel utilization is typically measured as an aggregate of the individual circuit utilizations and has a fixed bandwidth, while the individual circuits may be utilized by several users wherein each user may utilize an allocated portion of the circuit.
When a party needs to send and receive data over distances, the party (end user) enters into a service level agreement with a service provider to provide access to a data communications network. Depending on an individual end user's needs, the service level agreement may include provisions that guarantee certain minimum performance requirements that the service provider must meet. For example, if the end user expects to send and receive a certain amount of data on a regular basis, the end user may want the service provider to guarantee that a certain minimum bandwidth will be available to the end user at all times. The end user may want the service provider to guarantee that the average or minimum ratio of data units delivered by the network to data units offered to the network at the far-end is above a certain percentage and/or that the average or maximum transmission delays will not exceed a certain duration.
From a service provider's perspective, it is competitively advantageous to be able to demonstrate to potential and existing end users that the service provider is capable of meeting such network performance metrics. Thus, the capability to provide Quality of Service (QoS) analysis of network system performance at the service level, particularly in the context of network systems that share bandwidth between sites, is desirable from both an end user and service provider standpoint. Packet-switched architectures are evolving to support quality of service provisioning and the service level agreements which define the system performance requirements on a customer-by-customer basis.
Further, because of packet switching and new digital technology, there is no longer a need to have separate circuits for voice, data, and video, and it is foreseeable that delivery of several such forms of information will converge to “multimedia” being delivered on a common network over a single “pipe”. Moreover, parallels between commercial and military system requirements suggest that military and commercial infrastructures are also converging. For example, military tactical situations, such as delivering high speed data to a warfighter, require solutions analogous to those required in commercial contexts, such as business to consumer transactions.
As traditional circuit-switched communications systems transition to a “network-centric” architecture based on packet-switching, satellite communications (SATCOM) systems will likely be required to support such architectures and will be expected to support the end-to-end provisioning of services based on Quality of Service (QoS) metrics and customer Service Level Agreements (SLAs). Concurrent with this transformation of the terrestrial infrastructure is the evolution of SATCOM infrastructure to Ka-band frequencies. At Ka-band frequencies, the required antenna size is smaller than at lower frequencies, while more bandwidth is available to deliver more bits per second. In effect, the greater bandwidth and smaller antenna at Ka-band provides bigger “pipes” to smaller aperture antennas.
In terms of where commercial satellite communications are likely headed, the low-earth-orbiting (LEO) and medium-earth-orbiting (MSS) programs such as Iridium and Globalstar which were oriented toward the mobile satellite services have not been successful, which may impact rollout strategies for anticipated high speed data LEOs. On the other hand, broadband data trunking over geostationary (GEO) satellites exists today and is growing. The anticipated latency problems of TCP/IP (transmission control protocol/internet protocol) over satellite have been dealt with, and new bandwidth-efficient modulation techniques allow more bits/second to be delivered over the same frequencies. Thus, geostationary satellites using bandwidth efficient modulation at Ka-band present an opportunity to realize broadband over satellite communications.
While Ka-band offers a substantial increase in bandwidth, Ka-band carries a signal attenuation penalty associated with rain and other precipitation due to the inherent signal absorption characteristics of water at these frequencies. The current method for maintaining acceptable link performance in the face of rain-induced attenuation is a combination of large static power margins (i.e., putting enough power margin in the link in the first place so that sufficient power is received even during a rain event) and adaptive link power control (i.e., adaptively raising the power of the link in response to rain). Each of these approaches has drawbacks. The maintenance of large static power margins is not efficient, and adaptive link power control algorithms developed for X-band SATCOM may not be easily extensible to operation at the higher frequencies to which SATCOM is evolving (e.g., Wideband Gapfiller at Ka-band). Adaptive link power control requires an additional control system overlaid on the communications system to control the power level on a link-by-link basis. Such a link power adjustment control system is relatively complex and expensive and would be undesirable in a commercial communications system to mitigate downlink rain fades, where thousands or millions of customers may be covered by a satellite's footprint, each of which would require a corresponding power control link.
Thus, there remains a need for a seamless interconnection between terrestrial communications infrastructure and the satellite communications transport segment to support Broadband/Packet-switched communications (e.g., IP, TCP/IP, ATM) over geostationary satellite communications. In particular, there remains a need for a system which integrates with terrestrial infrastructures operating from service level agreements based on quality of service, and which provides high speed links adaptable to changing propagation effects that occur at Ka-band without requiring a complex control system overlay to provision services and manage performance.